Ideally you want to have enough space so that a womans high heel won't get caught but large enough for water to drain without collecting mid sized particulates.

If the PT decking is over 19% moisture content or feels very heavy for it's size it may be laden with moisture. If this is the case use a typical 16d common nail as your spacer... 1/8 ... as the deck dries it will widen the space to 3/16.

If the decking comes KD (kiln dried), has been sitting in the sun or feels light for it's size & you are installing in the summer heat, use upwards of a 1/4 inch spacer. These then will collect moisture in the cellular structure and close the gap to the 1/8 - 3/16.

In the winter months which is the most moisture laden time of the year for most locations, the stock will expand with the content closing the spacing when it is most crucial.

Again though you need to asses the moisture content during installation.

..and k2 you are even more correct with composites, it's extremely important to refer to the manufacturers specifications when spacing that decking as the polymers used expand almost exponentially. :)

They do have a tendency to over do it sometimes though, for example butted (end to end) decking for the most part (depending on the mean temp. at installation as well) requires a 3/16 to 1/4 spacer. I rarely do that only if there is absolutely no room for expansion.

i have built a few decks around my house, and have found that spacing the boards 'one nail' apart accomplishes many things. for me, i think it just plain looks better overall, 2, it really helps with drainage, and 3, ever sweep a deck that had all the boards butted up to one another? it's like sweeping a dirt floor.

also, as mentioned before, once these boards are down, they will move around when they get soaked/dry out, so better to give them some room to move around. also make sure to figure in a little pitch to the whole thing so water will better drain off of it.

MOST of the claims I'm called out on revolve around decks that had been spaced when they shouldn't have. Each case really is different. On decking with high moisture content (freshly treated), it's a good idea to butt the pieces up (as homebild recommended). On decking that hasn't been freshly treated and has close to equilibrium moisture content (that of your surroundings), then it should be spaced.

There's no hard and fast rule to this- it's all about moisture content. When we treat lumber, we can swell it as much as 8 to 10 percent. Now that CCA is history, we don't have to let freshly treated lumber languish on drip pads waiting for fixation to occur. That means that lumber yards who traditionally keep low inventories often have lots of PT around that is freshly treated and of high moisture content. The old rules don't apply anymore.